A fuel cell system for an environmentally friendly hydrogen fuel cell vehicle includes a fuel cell stack generating electrical energy through an electrical chemical reaction of reactive gas. A hydrogen supplier supplies fuel gas, that is, hydrogen, to the fuel cell stack. An air supplier supplies air containing oxidizing gas, that is, oxygen. A heat and water management system controls an operation temperature of the fuel cell stack, and performs a water management function. A fuel cell system controller controls the overall operation of the fuel cell system.
FIG. 1 is a diagram illustrating a configuration of a general fuel cell system. In a fuel cell system, a hydrogen supplier thereof includes a hydrogen storage (hydrogen tank) 21, a hydrogen supply line 22, a regulator (not shown), a hydrogen pressure control valve 23, and a hydrogen re-circulator. An air supplier of the fuel cell system includes an air blower 33 and a humidifier 34. A heat and water management system (not shown) of the fuel cell system includes a water trap, a motor-driven water pump (cooling water pump), a water tank, a radiator, etc.
High-pressure hydrogen supplied from the hydrogen tank 21 of the hydrogen supplier is supplied to the fuel cell stack 10 after being pressure-reduced to a predetermined pressure in the regulator (not shown) by the hydrogen pressure control valve 23 at an inlet side of a cathode. In this case, the pressure-reduced hydrogen is supplied to the fuel cell stack while controlling amount thereof through pressure control according to operation conditions of the fuel cell stack 10.
That is, the hydrogen passing through the regulator after being supplied from the hydrogen tank 21 is supplied to the fuel cell stack 10 after being pressure-controlled by the hydrogen pressure control valve 23 at the inlet side of the cathode. The hydrogen pressure control valve 23 adjusts hydrogen pressure-reduced by the regulator to a pressure appropriate for a stack operation condition.
In the hydrogen re-circulator, an ejector or re-circulation blower 25 is connected to a re-circulation line 24 at an anode of the fuel cell stack 10 to re-circulate unreacted hydrogen, which is not used by the anode of the fuel cell stack 10, and to re-use of hydrogen.
The air supplier humidifies air supplied by the air blower 33 through the humidifier 34, and then supplies the humidified air to the fuel cell stack 10 via an air supply line 35.
Nitrogen contained in the air supplied to the cathode of the fuel cell stack 10 in accordance with the operation of the fuel cell stack 10 and water (water and vapor) produced at the cathode of the fuel cell stack 10 pass through an electrolytic membrane disposed in the fuel cell stack 10, and then move to the anode of the fuel cell stack 10.
The nitrogen reduces a partial pressure of the oxygen, thereby degrading the performance of the fuel cell stack 10. The produced water blocks a flow channel of a separation plate channel, thereby restricting movement of the hydrogen. As the amount of the foreign matters, e.g. nitrogen, water, and water vapor fed to the anode through the electrolytic membrane within the fuel cell stack 10 increases, the amount of hydrogen at the anode is reduced, and thus, reaction efficiency decreases.
Therefore, it is necessary to remove nitrogen from crossing-over air and liquid droplets in the channel through periodic purging of the anode in order to secure stable performance of the fuel cell stack 10.
That is, a purge valve 40 is connected to an outlet line of the anode of the fuel cell stack 10, for purge of hydrogen. The hydrogen purge is performed by periodically opening the purge valve 40 at intervals of a predetermined period during travel of the vehicle in order to increase purity of hydrogen in the fuel cell stack 10. The purge valve 40 is also opened to purge the hydrogen in the fuel cell stack 10 upon shutdown (start-off) of the fuel cell system or start-up of the fuel cell system. In this case, the purged hydrogen is exhausted outwards through an exhaust line 41 at an outlet side of the anode in the fuel cell stack 10, exhaust lines 36 and 37 at the side of the cathode, and humidifier 34.
As the hydrogen from the anode is periodically exhausted, foreign matters such as nitrogen or moisture is removed from the separation plate channel in the fuel cell stack 10 through exhaust thereof. Accordingly, it may be possible to achieve an increase in hydrogen concentration and hydrogen use rate, and an enhancement in gas diffusion and reactivity.
The purge valve 40 is an electromagnetic control valve, which is periodically opened or closed in accordance with a command from the fuel cell system controller (not shown) for hydrogen concentration management. When the purge valve 40 is opened, the foreign matters such as moisture or nitrogen present in the fuel cell stack 10 are discharged to the atmosphere through a vehicle exhaust port connected to the exhaust line 37.
When the purge valve 40 is opened during driving of the vehicle, the hydrogen may be exhausted to the atmosphere via the exhaust lines 36 and 37 by a pressure difference between the anode (relatively high pressure) and the cathode in the fuel cell stack 10. In this case, the foreign matters are discharged together with hydrogen, and thus, a desired output of the fuel cell stack 10 may be maintained.
FIG. 2 is a diagram of an air flow in a fuel cell system. Referring to FIG. 2, air cutoff valves 32 and 38 are installed at an air supply line 31 at an inlet side of the air blower 33 and the exhaust line 37 at an outlet side of the humidifier 34, respectively.
Humid air discharged through a cathode-side outlet of the fuel cell stack 10 and the exhaust line 36 in the configuration of FIG. 2 is exhausted outwards through the exhaust line 37 together with hydrogen discharged through the purge valve 40 after passing through the humidifier 34.
In addition, dry air supplied by the air blower 33 is humidified after receiving moisture from humid air discharged from the cathode of the fuel cell stack 10 through the exhaust line 36, and is then supplied to the cathode of the fuel cell stack 10 via the air supply line 35.
Since the air cutoff valves 32 and 38 are installed at the inlet side of the air blower 33 and the outlet side of the humidifier 34, respectively, air flows along a path including the air cutoff valve 32, air blower 33, humidifier 34, stack (cathode) 10, humidifier 34, and air cutoff valve 38.
Both air cutoff valves 32 and 38 are controlled to be in an opened state in order to allow air to flow along the above-mentioned path.
Hereinafter, problems encountered in the conventional fuel cell system will be described.
First, both air cutoff valves 32 and 38 are closed upon shutdown of the fuel cell system. In this state, hydrogen purged in accordance with the opening of the purge valve 40 is left not only in the humidifier 34 after passing through the anode-side exhaust line (“41” in FIG. 1) and the cathode-side exhaust line 36, but also in the lines 31, 35, 36, and 37 between the air cutoff valves 32 and 38, the air blower 33, the humidifier 34, and the cathode of the fuel cell stack 10.
When such air is left within the system, performance degradation and durability degradation of the fuel cell stack 10 may occur.
In fuel cell vehicles, safety may be secured without a risk of fire when the concentration of hydrogen exhausted outwards does not exceed a predetermined level. In general, the concentration of hydrogen exhausted from fuel cell vehicles is regulated to peak at 8% or less and have an average of 4% or less for 3 seconds.
In order to meet such Global Technical Regulations (GTRs) through reduction of the concentration of exhausted hydrogen, motor companies use a method in which hydrogen is accumulated in a dilution device, and is then exhausted after being diluted through injection of air or a method in which hydrogen is combusted.
However, when the time from shutdown of the fuel cell system to a next start-up is lengthened (when the shutdown time of the fuel cell stack is lengthened), there may be a phenomenon in which hydrogen in the fuel cell stack 10 crosses over the cathode through the electrolytic membrane. As degradation of the fuel cell stack 10 becomes severe, the amount of hydrogen crossing over the cathode increases.
The hydrogen purged during start-up, hydrogen crossing over the cathode through the electrolytic membrane in the fuel cell stack 10, and hydrogen accumulated in the humidifier during shutdown are ejected by air supplied from the air blower 33, and thus, are exhausted through the exhaust line 37. In this case, hydrogen is pushed into the exhaust line 37 without being mixed with air, thus, increasing the concentration of hydrogen and a fire risk. Furthermore, it may be impossible to meet Global Technical Regulations (GTRs).
In conventional cases, a reduction in hydrogen concentration is achieved through change of software in a controller. However, such hydrogen concentration reduction has limitations. When degradation of a stack becomes severe, a phenomenon in which hydrogen in the fuel cell stack 10 penetrates into a cathode increases, and thus, the concentration of hydrogen exhausted outwards increases. Furthermore, it may be impossible to meet the desired regulations.
When shutdown (start-off) occurs at ambient temperature of 5° C. or less, the air blower 33 is driven to discharge water in the fuel cell stack 10 to the exhaust lines 36 and 37 in order to prevent water in the fuel cell stack 10 from freezing. Thus, a cold start at a temperature below the freezing point of water in winter may be achieved.
In this case, a portion of water discharged from the fuel cell stack 10 is fed back to the fuel cell stack 10, and thus, efficient water removal may not be achieved. When residual water in the fuel cell stack 10 freezes in winter, a start-up time taken for the cold start may increase or a start-up failure may occur.